1. Field of the Invention
The invention relates to a method of preventing deposition of oil on steel mill scale particles, and, more particularly, to a method of preventing deposition of oil by chemical treatment in steel mill flume water.
2. Description of the Prior Art
Oily mill scale presents a tremendous waste handling problem for the steel industry due to the vast tonnage of mill scale produced at each hot rolling mill location. A cost-effective incineration process to bum off the oil is prohibited under a number of federal regulations (i.e., blue smoke, hydrocarbon release). Currently, there are no chemical or mechanical programs that remove the oil from the mill scale with reasonable cost to the steel industry.
In a typical hot rolling mill, as shown in FIG. 1, the steel ingot is rolled for shaping and thinning, and water is sprayed onto the steel to cool it and to knock off the iron oxide scale that forms as oxygen reacts with the steel surface. This mill scale falls through a grating below the steel stands and into a flume where water is collected. A variety of oils, greases, and hydraulic fluids (hereafter called oil) utilized for lubrication of mechanical equipment also drop into the flume water. The mill scale is carded in the flowing water in the flume to a scale pit where the oil, scale, and water are separated. However, the mill scale is now oil-wet as a result of contact with the oil in the flume water trench and scale pit. In some cases, mill scale may contain up to 10% oil.
Steel mills consider the recycling of mill scale highly desirable, since mill scale provides higher iron content than iron ore. However, there are problems with direct use of oily mill scale in the steelmaking process. In typical operations, iron ore and other particulate iron sources such as mill scale are agglomerated in a sintering process to reduce air emissions of particulates from the blast furnace. Agglomeration is accomplished in the sinlet plant by exposing the iron-containing feed to elevated temperatures, so that the particles fuse together to form agglomerates. The oily mill scale is thus exposed to a temperature gradient, resulting in some vaporization of the oil and release to the off gases. There is concern over hydrocarbon (oil) emissions from the sinter plant because of the need to comply with various air pollution regulations. Oily mill scale can also cause processing problems in the sinter plant, such as fouling of air filters in the "baghouse" and, in extreme cases, back-fires in the furnaces.
Therefore, there is a limit on the amount of oil that can be tolerated in the sintering operation. The limit for oil on mill scale is site specific, depending on local regulations and on other hydrocarbon emissions from within the mill and also on the amount of oily mill scale that is being processed. In some mills, oil content of mill scale must be less than 1%, while other mills may utilize mill scale with 2% or more oil. In general, however, reduced oil content mill scale may allow the steel mill to recycle more oily mill scale daily, reducing the amount of iron ore being purchased.
A cost-effective incineration process to burn off the oil from the mill scale is prohibited under a number of federal air pollution regulations (i.e., blue smoke, hydrocarbon release). Currently, there are a number of chemical or mechanical processes to remove the oil from the mill scale, but these processes suffer from high capital and operating costs. In a mill producing 100,000 tons of oily mill scale per year, annual costs for oil removal would be over $1,000,000.